March and April showers may bring May flowers, but for growers of Thompson seedless, Flame Seedless, Redglobe or Grenache variety grapes, those spring rains also bring on the disease known as phomopsis cane and leaf spot.
Unfortunately, cultural or biological control of this disease is marginal and some type of chemical control is usually necessary. Many may question the use of various chemicals for disease control on the basis of food safety, however, the application of chemicals at this stage of growth avoids all contact with the fruit. This is because the flower of the grape, which becomes the fruit, is covered with a cap or calyptra. At bloom, the calyptra is shed onto the ground hence avoiding all chemical contact with the fruit.
The spread of this disease (formerly called dead-arm) is associated with rain during bud break and subsequent early shoot growth. Spores are released in large quantities from previous infection sites on diseased canes, spurs and bark at bud break and splashed by rain onto the young growing shoots.
Once growth has reached 15-18 inches the canopy forms an umbrella and helps prevent the splashing effect, hence decreasing the need for chemical applications. The amount of disease depends on the frequency, timing and amount of rain. Hence the disease is seasonal in its occurrence.
We have been looking at old and new chemistries the past five years to find a chemical that provides effective control of this disease. The chemicals fall into three major categories: dormant, foliar contact protectants and foliar systemic protectants.
Only one material, lime sulfur, has been effective in controlling this disease when applied to dormant (prior to budbreak) vines. The other registered materials act as foliar protectants and need to be applied to the foliage after bud break (during the highest potential infection period) but before the rain occurs. Various copper/sulfur materials (alone or in combination), ziram, captan, mancozeb or maneb are all foliar contact protectants and need to be applied before spring rains and then re-applied after significant rainfall or significant shoot growth for continued protection in the event of another rain event.
The strobilurins (Abound, Sorvan, Flint and Pristine) are classified as foliar systemics and have the advantage of not washing off in wet weather. Growers should protect grapevine foliage if rain is forecast after this period. It is best to look at the head of the vine to judge for bud break rather than the tips of the canes. It is important to protect the renewal shoots for next year's production.
In using these materials it would probably be better to use the less expensive foliar contact protectants in the early stages of growth saving the more expensive foliar systemic materials for use when shoots are at least 4-6 inches. Systemics would also be better if a series of storms are predicted making it difficult to re-enter the vineyard between storms to re-apply materials.